A method of manufacturing a semiconductor device including forming an ONO
film on a semiconductor substrate and a hard mask layer on the ONO film,
forming a trench by etching the hard mask layer and the ONO film on a
field region of the semiconductor substrate using a photo etch process
and etching the field region of the semiconductor substrate, and forming
a device separator at the trench. The method also includes exposing the
ONO film by removing the hard mask layer on the ONO film, and leaving the
ONO film only on a prospective SONOS gate in a cell region of the
semiconductor substrate and removing the ONO film the remainder region
thereof. The method further includes forming a gate oxide film on the
semiconductor substrate at an outside of the ONO film, and forming a gate
electrode on the gate oxide film and the ONO film, respectively.